Chalcones (1,3-diaryl-2-propen-1-ones) are open chain flavonoids that have an enone moiety between two aromatic rings. As recently reviewed (Batovska D and Todorova I, 2010; Patil C et al., 2009; Go M et al., 2005), different families of natural chalcones have been isolated from plant extracts and characterized as having relevant biological properties such as antioxidant, cytotoxic, anticancer, antibiotic, antiinfective, hypoglycaemic, and anti-inflammatory activities.
Chalcone derivatives are used, or are under development, for medical uses, as well as like food additives and cosmetic formulation ingredients, and the pharmacological potential of chalcone derivatives is considered to be not yet fully exploited. In that respect, libraries of synthetic chalcone derivatives have been generated and screened using animal models, cell-based assays and/or biochemical assays, in order to establish structure-activity relationships and identify compounds having improved biological properties (such as target specificity, potency, bioavailability, and/or safety) or chemical features (such as stability or lipophilicity). Thus, chalcone is considered as a template molecule that can be adapted to desired activities by introducing specific chemical moieties and/or conformational restraints (Katsori A and Hadjipavlou-Litina D, 2009; Jamal H et al., 2008; Chimenti F et al., 2009; Sivakumar P et al., 2009; Henmi K et al., 2009; Srinivasan B et al., 2009; Patil C et al., 2009; Rao G et al., 2009; Reddy M et al., 2008; Alberton E et al., 2008; Romagnoli R et al., 2008; Gacche R et al., 2008; Liu X et al., 2008a; Hachet-Haas M et al., 2008; Chiaradia L et al., 2008; Cabrera M et al., 2007; Jung S et al., 2006; Go M et al., 2005; Ansari F et al., 2005; US20070092551).
In particular, there are several examples of natural or synthetic chalcone derivatives that contain at least an aromatic ring with substitutions on adjacent carbon atoms. For example, Licochalones, Derricin, and other natural chalcone variants show antibacterial or antiparasitic activity, cytotoxic activity against human cancerous cells, or proapoptotic activity on endothelial cells (Cunha G at al., 2003; Yoon G et al., 2005; Ghayur M et al., 2006; Ogawa Y et al., 2007; Matsuura M et al., 2001; Na Y et al., 2009; Tsukiyama R et al., 2002; Zhu X et al., 2005). Libraries of synthetic Licochalcone variants or conjugates have been produced and tested in various models (Kromann H et al., 2004; Yoon G et al., 2009; Liu X et al., 2008b).
The biological activities of natural or synthetic chalcone derivatives that have multiple substitutions on one or both phenyl groups have been described, such as insulin-mimetic action (US20070092551), anti-inflammatory activities (WO 01/98291), or inhibition of angiogenesis (WO 01/046110). Chalcone derivatives containing substituent groups on at least three adjacent (or consecutive) carbon atoms of a phenyl ring (such as those described in WO 04/005233, WO 05/073184, WO 07/147,879, WO 07/147,880, and U.S. Pat. No. 7,524,975) are activators of one or more Peroxisome Proliferator-Activated Receptors (PPARs), a family of nuclear receptors that are therapeutic targets, in particular for treating metabolic or neurodegenerative disorders (Akiyama T et al., 2005; Gross B and Staels B, 2007).
Generally, synthetic chalcone derivatives are produced by a Claisen-Schmidt condensation reaction of an aldehyde with a ketone, but other approaches are possible, such as palladium-catalysed reactions (Patil C. et al., 2009; Katsori A. and Hadjipavlou-Litina D., 2009). However, the acidic nature of the obtained compound and the frequent presence in the reaction medium of secondary products and unreacted starting materials require additional steps of purification and/or specific approaches, such as microwave irradiation, that result in a significant reduction in the yield and/or make difficult the later modifications of these compounds. In fact, chalcone derivatives can be used as starting materials for producing other classes of compounds such as flavonoids or pyrazoles.
Phase transfer catalysis, which is considered as a reliable strategy for the asymmetric synthesis of organic compounds in simple experimental conditions, mild reaction conditions, and for large-scale preparations, has been used to the modify chalcone derivatives through condensation of intermediate compounds, as well as epoxidation or Michael addition of chalcone derivatives (Ooi T and Marouka K, 2007; Song G and Ahn B, 1994; Li J and Liu X, 2008; Rao G et al., 2009).
The synthesis and/or further modification of chalcone derivatives that contain at least a phenyl ring with substituent groups on adjacent carbon atoms of the ring can be inefficient due to steric hindrance. Alternative synthetic strategies for producing such chalcone derivatives, in particular by either S-alkylation or O-alkylation, have been described in the literature cited above and elsewhere (WO 05/005369, WO 04/056727). However, the need for novel methods allowing the efficient production of chalcone derivatives that have multiple substitutions, and in particular on adjacent carbon atoms of a phenyl ring, is still clear and urgent.